Is it even worth it for me to crunch CPU WUs if I can crunch GPU ones? Are there some WUs that *need* to be processed by a CPU, or are those WUs only made for people without GPUs essentially? Basically are these resources better spent just going towards a CPU only project (and might they take away what Milkyway gets on the GPU?). Just - is there honestly a reason for me to crunch CPU WUs for Milkyway if I can crunch GPU ones as well?

Short answer: probably not.
Long answer: It depends on how much it takes away from the CPUs feeding the GPUs. My rule of thumb is 1 (dedicated) core per GPU. I found that 4x Radeon 6950's being fed by a quad-core i7, slowed their throughput more than the microscopic credit earned by running CPU-only tasks in tandem.

Does depend on the GPU somewhat, older GPUs don't really need a dedicated CPU core, my HD 4870 used to run at nr 100% load with all my CPUs running Folding or Asteroids. My 5850 though is at ~80% load.
However my rigs totally output between the 2 projects is higher leaving all cores crunching.